Conductive materials, including conductive metal particles and metal-plated resin particles, are currently be used in electrical connections between microelectrodes of electronic devices, e.g., between ITO electrodes and driver IC circuits, between driver IC chips and circuit boards and between micro-pattern electrode terminals in liquid crystal display (LCD) panels. In particular, conductive particles that include metal coated onto deformable and recoverable polymer resin particles may be used in electrical connections and may provide reliable interconnectivity for long time periods. In recent years, as the pitch of circuits has become finer and the area of bump electrodes has become smaller, the conductivity and electrical reliability of conductive particles may be even more important to providing a suitable electrical connection.
A complex conductive particle may be prepared by coating a conductive metal, e.g., gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), nickel (Ni), cobalt (Co), tin (Sn) and indium (In), onto the non-conductive surface of a polymer resin particle by plating. The electrical and physiochemical properties of the final particle may be varied according to the kind and number of metals being introduced. A Ni/Au complex metal layer has previously been used as the metal layer in a conductive particle used in an anisotropic conductive film (See, e.g., Japanese Patent Laid-Open Nos. 1999-329060 and 2000-243132). A Ni/Au complex metal layer may be desirable because nickel may be readily formed into a thin metal layer by electroless plating, while Au may provide the particle with excellent conductive properties. Thus, a conductive particle having a Ni/Au complex metal layer may exhibit stable electrical connection properties at a connection site of a semiconductor or other mounting device.
Japanese Patent Laid-Open No. 2000-243132 describes a Ni/Au complex plating layer formed by electroless plating a substantially indiscrete Ni layer on a polymer resin particle, and then forming a Au layer on the Ni layer by substitution plating. The phrase “substantially indiscrete Ni layer” as used herein refers to a plating layer having a thickness of 5 nm or more formed by deposition of Ni particles during plating. When taking into account plating adhesion to the base particles, the Ni layer commonly has a thickness of about 50 nm to about 70 nm. The substantially indiscrete Ni layer may facilitate the introduction of the Au layer. However, when a conductive particle having a Ni plating layer within this thickness range is interposed between electrodes and compressively deformed, peeling between the Ni layer and the polymer resin particle may occur. When compressive deformation is continued, peeling may lead to the rupture of the Ni layer, resulting in decreased electrical connection. This may occur because the Ni layer is harder and less flexible than the polymer particle.
Various efforts have been made to alter the properties of Ni layers formed on polymer resin particles. For example, Japanese Patent Laid-Open No. 2507381 describes conductive particle having a Ni layer coated onto a resin particle, wherein the Ni layer has a phosphorous content of between 1.5 to 4 weight percent. Such particles are described as having increased flexibilty and adhesion relative to lower concentrations of phosphorus and increased conductivity under high temperature and humidity conductions and improved corrosion resistance relative to Ni plating layers with higher levels of phosphorous. However, such conductive particles may aggregate due to the magnetism generated by the low phosphorous content. Further, during disintegration for dispersing the aggregated particles, peeling and rupture of the plated layer may occur.
Japanese Patent Laid-Open No. Hei 7-118866 describes a conductive particle having a nickel-phosphorous alloy layer with 7 to 15 weight percent phosphorous plated onto the surface of a spherical core material by electroless plating. By incorporating 7 to 15 weight percent of phosphorous, aggregation during plating or re-aggregation during dispersion may be reduced, which may improve the dispersibility of the conductive particles. However, the conductivity of the particles may be decreased due to the high phosphorous content of the plating layer.
Thus, a specific content of phosphorous cannot ensure that a conductive particle has both desirable adhesion and desirable conductivity. Further, the flexibility of the nickel-phosphorous plating layer may be somewhat dependent on the electroless plating conditions. In addition, there is no clear trend regarding the effect of the phosphorous content on the properties of conductive particles that include a nickel-phosphorous alloy and another metal. However, it is known that the electrical resistance of a conductive particle generally increases as the amount of phosphorous in the nickel-phosphorous layer increases.
Therefore, it would be desirable to provide highly reliable conductive particles for anisotropic interconnection that include a Ni/Au complex plating layer with a controlled phosphorous content formed onto the surface of polymer resin particles. It would further be desirable for the polymer resin particles to be relatively uniform in size, for the Ni/Au complex metal plating to adhere sufficiently to the surface of the polymer resin particles, and for the particles to have desirable conductive properties.